If you buy a used muzzleloader there are several things to know about.
Rusty Bores
The biggest problem will be a rusty bore.
This is caused by the previous owner not cleaning the gun right after shooting it.
I've seen everything from light discoloration to deep pits caused by this.
Light discoloration or even some slightly rough areas that leave the rifling undamaged won't be an issue when it comes to shooting it.
Use the presence of light rusting to reduce the asking price.
Deep pits, "rough as a cob" and damaged rifling signifies a totally ruined barrel.
Pass on a barrel that has severe rust damage.
Bulged or "ringed" barrels:
These are fairly common and because of the thick wall on a muzzleloading rifle, usually the bulge is not noticeable just by looking at it.
It's a good idea to have a cleaning jag that is sized for the caliber of gun your looking at in your pocket. You also need a few pieces of lightly oiled cloth for a patch.
Most ramrods have a threaded hole in the end to attach the cleaning jag.
Using the correct jag, run the cloth patch down the bore to the breech.
As you do this, get a feel for how much force it takes.
If the force is constant for the full length of the bore, the barrel is fine.
If the force suddenly gets "easy" and then returns to the previous force, the barrel is bulged.
A bulged barrel may or may not be dangerous to shoot but it will never shoot accurately. Pass on a gun that has this problem.
Loaded guns
When your doing the bulge test, when the ramrod stops at the bottom of the barrel, place your thumbnail on the rod, flush with the muzzle.
Keeping your thumbnail at its location, remove the ramrod and patched jag.
Now, lay the ramrod alongside the barrel and notice the location of the distant face of the cleaning jag.
If the barrel uses a side drum or a vent hole drilled directly into the side of the barrel, the front of the cleaning jag should be even with or past that feature.
If the barrel uses a "snail" breech like most modern factory caplocks and some flintlocks the end of the jag should be within 5/8 inches of the place where the barrel material stops and the breech plug material starts.
If the end face of the jag is more than 5/8" forward of the breech plug or side drum/vent hole there is a good chance the barrel is loaded.
Happy shopping.